parrent armature bone to 3 vertecies of a different mesh
To strictly answer the question: Easiest thing is to parent an empty to the mesh vertices, then copy transforms from that empty.  (And probably limit scale, because vertex triangle parenting leaves you with some pretty gigantic skew problems that you probably don't want...)
Alternative, without any empty, is to copy location from a vertex group marked vertex, damped track another vertex group marked vertex, and then locked track a third vertex group marked vertex.  (Copy transform or rotation from vertices is never a good idea.)
However, most of the time people ask this question or one like it, they're trying to do something impossible.  They're trying to acquire this position and then use it to further deform the same mesh that they're &quot;parented&quot; to.  You can't do that-- it's a dependency loop.  The mesh depends on the bone which depends on the mesh which depends on....  The end goal of people asking this may be achievable, but this particular technique is not a means of achieving it (and the technique that works is likely to be complicated, possibly involving dummy copies of the mesh.)